Phosphorus: A Killer Preservative

Chemical Fertilizes Algae, Fish Suffocate

Phosphorus damages rivers by encouraging "algae blooms," brownish expanses of algae on the water's surface, which use up oxygen needed by fish, shellfish and the microscopic marine life they feed on.

Here's how that process works:

Algae is basically made up of three elements: carbon, nitrogen and

phosphorus.

Some forms of algae are capable of "fixing," or removing, both nitrogen and carbon from the air. Phosphorus is typically the major raw material that is in limited supply.

Discharging high levels of phosphorus and nitrogen into rivers is like putting fertilizer on a lawn. The extra nutrients enrich the water, creating fertile conditions that can lead to abnormal algae growth if water temperatures and salinity levels are also at optimum levels.

Once algae growth takes off, huge brownish-looking blooms can stretch across the water's surface for as much as several hundred feet, which can have several detrimental effects on the river's normal food chain.

For example, the algae cover reduces the amount of sunlight available to other organisms in the river. Also, some forms of algae are toxic to other forms of marine life.

Most importantly, the algae blooms create tremendous swings in the amount of dissolved oxygen that is available to other organisms. During sunny days, algae blooms actually produce additional oxygen for the environment through the process of photosynthesis. But on cloudy days, and especially at night, the algae uses up tremendous amounts of dissolved oxygen in the water.

On a bright summer day, dissolved oxygen levels in an algae-covered area could be as high as 11 or 12 milligrams per litre of water. On some nights, the dissolved oxygen levels in that same area could drop to near zero, making it extremely difficult for some organisms to survive.

Normal dissolved oxygen levels average about 4 milligrams per litre.

If the oxygen swings are severe enough, fish kills can occur. In come cases the fish are able to leave the area of the algae blooms. But organisms with limited mobility can eventually die from lack of oxygen, depending on the size and life span of the algae cover.

Perhaps the most damaging result of algae growth and the subsequent oxygen depletion is the effect on other forms of microscopic plankton found floating in water. Plankton are used as food by nearly all aquatic animals. When some forms of phytoplankton form algae and deplete oxygen supplies, other forms of plankton are often killed by the resulting lack of oxygen. That severely limits the types of marine life that can survive in the area of the algae bloom.